1. Field of the Invention
This invention relates to an optical storage medium drive apparatus for recording information on an optical storage medium and reading out the recorded information from the optical storage medium by using a light beam.
2. Description of the Prior Art
Optical information recording has been developed as a high density recording method. The optical storage medium is in the form of disk or card. The conventional optical storage medium drive apparatus has a light source such as a laser for emitting a light beam, an optical system for guiding the light beam, a focus lens for focusing the light beam on the surface of the storage medium, and a light detector for converting the light beam reflected from the surface of the storage medium into an electric signal.
In the recording mode, a modulated strong light beam is emitted from the light source and focused on the surface of the storage medium to change the physical state of the focused spots, thereby recording a information in the form of a dot pattern on the surface of the storage medium. The size of each dot is 1-2 .mu.m, and thus a recording density of more than 4.times.10.sup.7 bits/cm.sup.2 can be obtained easily.
In the reading mode, a weak constant light beam is emitted from the light source and focused on the surface of the storage medium. A light beam reflected from the surface of the storage medium is modulated by the recorded dot. The reflected light beam is guided by the optical system to the light detector and converted by the light detector into an electric signal.
In the case of the magnetooptic storage medium, a information is recorded in the form of polarity variations of magnetic domains which are caused by being heated by the strong light beam. A linearly polarized light beam, when reflected from the surface of the storage medium, changes its polarizing angle according to the polarity of the magnetic domain on which the light beam is incident. This effect is known as the magnetooptic Kerr effect.
A conventional optical storage medium drive apparatus is disclosed in Japanese Laid-Open Patent Application No. 50-34507. The apparatus disclosed in this reference comprises a hollow arm support, a hollow arm turnable about a center axis of the hollow arm support, a scanning unit mounted in the hollow arm, and a light source unit mounted on a fixed member. An optical path between the light source unit and the optical storage disk is formed in the hollow arm support and the hollow arm.
Another conventional optical storage medium drive apparatus is disclosed in U.S. Pat. No. 3,898,629. This reference also shows a hollow shaft of a motor and a hollow arm having an optical system mounted therein.
Since the optical system for forming an optical path between the objective lens and the light source or the light detector contains some reflectors for changing the direction of the light beam, there occurs a problem of retardation, i.e. a phase delay of the light wave. In other words, the polarizing angle of the light beam changes when reflected by each of the reflectors, so that the linearly polarized light changes to an elliptically polarized light. Further, since the polarization of the light varies according to the rotation angle of the reflectors, the elliptical shape of the elliptically polarized light varies according to the tracking position of the light beam. The retardation of the reflected light causes a deterioration of the read-out information.
In the case of using the magnetooptic storage medium as described above, the effect of retardation is serious. This is because the data recorded on the storage medium are read in the form of small changes in polarization of the reflected light. The signal to noise ratio (S/N) of the read data is seriously deteriorated due to the retardation.